H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 17)

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Figure 1 Establishing a client access
Scanning
A wireless client gets surrounding wireless network information in two ways, passive scanning and active
scanning. With active scanning, a wireless actively sends a probe request frame, and gets network
signals from received probe response frames. With passive scanning, a wireless client gets wireless
network information by listening to Beacon frames sent by surrounding APs.
Actually, when a wireless client operates, it usually uses both passive scanning and active scanning to
get information about surrounding wireless networks.
1. Active scanning
When a wireless client operates, it periodically searches for (scans) surrounding wireless networks.
Active scanning falls into two modes according to whether a specified SSID is carried in a probe
request.
{
A client sends a probe request with no SSID (the SSID IE length is 0): The client periodically
sends a probe request frame on each of its supported channels to scan wireless networks. APs
that receive the probe request send a probe response, which carries the available wireless
network information. The client associates with the AP with the strongest signal. This active
scanning mode enables a client to actively get acquainted with the available wireless services
and select to access the proper wireless network as needed. The active scanning process of a
wireless client is as shown in Figure 2 .
Figure 2 Active scanning (the SSID of the probe request is null, or, no SSID information is carried)
AP 2
Client
AP 1
Prob
e req
uest
(wit
h no
SSI
D)
Probe r
equest (
with no
SSID) Probe R
espons
e
Prob
e Re
spon
se